The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to multi-slice, multi-angle acquisition of NMR data.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The concept of acquiring NMR imaging data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantage of echo-planar imaging ("EPI") are well-known, and there has been a long felt need for apparatus and methods which will better enable EPI to be practiced in a clinical setting. Other echo-planar pulse sequences are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,716,369; 4,355,282; 4,588,948 and 4,752,735.
A variant of the echo-planar imaging method is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3, 823-833 (1986) entitled "RARE Imaging: A Fast Imaging Method for Clinical MR." The essential difference between the RARE sequence and the EPI sequence lies in the manner in which echo signals are produced. The RARE sequence, utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill sequence, while EPI methods employ gradient recalled echoes.
Both of these fast imaging methods involve the acquisition of multiple spin echo signals from a single excitation pulse in which each acquired echo signal is separately phase encoded. Each pulse sequence, or "shot," therefore results in the acquisition of a plurality of views. However, a plurality of shots are typically employed to acquire a complete set of image data when the RARE fast spin echo sequence is employed. For example, a RARE pulse sequence might acquire 8 or 16 separate echo signals, per shot, and an image requiring 256 views would, therefore, require 32 or 16 shots respectively.
In nearly all two-dimensional clinical scans the NMR data are acquired for a plurality of slice images. The acquisition of many slices can be achieved without increasing the scan time because much of the scan time is otherwise wasted waiting for the longitudinal magnetization to recover. By "interleaving" the pulse sequences for different slices within each TR period, this otherwise idle time is used to acquire additional slices.
In most clinical scans the slices acquired during an interleaved scan are disposed one next to the other in parallel planes. However, there are clinical applications in which the slices are not parallel. On application, for example, is imaging the spinal column where slices through various vertebrae are oriented at different angles due to the curvature in the spine. As disclosed in U.S. Pat. No. 4,871,966, to acquire data in an interleaved scan from slices oriented at different angles, it is necessary to change the imaging gradients during the scan to rotate the separate slices to the required orientations. One of the difficulties with such multi-angle, interleaved scans is that flow artifact suppression techniques such as that disclosed in U.S. Pat. No. 4,715,383 cannot be used with maximum effectiveness. This method employs a saturation rf pulse prior to each set of interleaved pulse sequences to suppress the signal from flowing spins "up-stream" from the image slices. For maximum effectiveness, the saturation band should be virtually contiguous with the set of image slices and this in not possible when the interleaved slices are oriented at different angles and spaced apart in different groups.
Another clinical application in which multiple-slices are acquired at different angles is imaging of the temporomandibular joint (TMJ). In this case the separate slices may intersect in their field of view, and if they are all acquired in the same TR period as taught in U.S. Pat. No. 4,871,966, spins at the intersections will become saturated and their NMR signals will become reduced in amplitude.